The formation of gas bubbles in the body resulting in gas micro embolism or decompression sickness ("bends") can cause severe pain, injury or death. This condition can occur during rapid reduction in the ambient pressure associated with termination of hyperbaric oxygen treatment, ascent of divers, pilots undergoing high-altitude high-g maneuvers or astronauts undergoing extra-vehicular activities. It can also arise from the direct injection of gas into the body by aqueous oxygen injection to alleviate local ischemia or the formation of a gas pocket for laparoscopic surgery. A device capable of continuous non-invasive detection of gas bubbles would greatly contribute to the prevention of injury and increase the safety and efficiency of such procedure. The present application proposes to develop a novel instrument based on an acoustic approach for non-invasive detection and characterization of bubbles in biological fluid and tissues/organs. The proposed method is based on a technique recently developed for the measurement of bubble distributions in liquids that consists of emitting and receiving acoustic bursts at different frequencies. Based on the sound speed and attenuation as functions of frequency, the number and size of bubbles in the acoustic path are computed. This effort proposes to develop a similar technique for measurement of bubbles in the blood stream and inside tissue and fluid cavities (e.g. synovial cavity) which are complex non- Newtonian media. Extensions to the numerical model to account for viscoelastic effects (e.g. tissue) and complex geometries (e.g. knee joint) will be developed, validated and integrated into the current system. Experiments will be conducted to test and validate the enhancement to the numerical model in non- Newtonian media such as artificial tissue and joint modeled using common phantoms. A detailed plan for future phase animal experiment will then be developed.